For the preparation of such low viscosity suspensions, use is made of axially closed pump systems forming individual chambers, e.g. counter-rotating twin screws, such as screw spindle pumps. Such pumping or mixing systems are able to build up high pressures in the case of very small chamber-casing clearances but, due to their kinematics, lead to a high degree of wear, which on the one hand increases maintenance costs and on the other hand over a period of time leads to a drop in the pressure build-up capacity. When it is necessary to obtain a specific pressure build-up capacity, the mixing and compounding efficiency of such counter-rotating screws is inadequate. In addition, the speeds of counter-rotating screw arrangements are relatively low and consequently so is the attainable throughput, based on the machine size.
Pumps without forced delivery, such as co-rotating screws have a high mixing efficiency and are subject to lower wear, but in the conventional manner cannot be used for building up the starting pressure required for certain processes in the case of low viscosity suspensions, because the pressure build-up capacity of such pumps is dependent on the viscosity of the medium to be delivered, apart from the screw geometry and length.
Solutions with continuously operating mixers and an additional booster pump are irrational and are subject to considerable limitations with regards to the various operating requirements and problems which occur. German Patent Specifications Nos. 972895 and 2850121 disclose the use of screw conveyors for supplying solids by means of such a conveyor to a process which is taking place under pressure. However, due to the consistency of the materials to be supplied, the pressure build-up then constitutes no problem. A gas seal between the conveying means or mixing means and the reaction chamber is achieved by a material plug formed at the end of the screw conveyor, which is continuously ejected into the reaction chamber, where it is dispersed again.